Blue Sky

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Hi,
I have a plan which I'm going to enlarge by 45% (49" span to 71" ) and my question is this: by how much, if any, should I increase the wood thicknesses?

Or, to put it another way, what thicknesses of wood would you use for a 71" span sport plane? All built-up wings, sheet & former fus, etc.

Oh, btw, it's a twin design... 49" version flew on two .10's, I was thinking 71" version to fly on two ASP .36's (Chinese, bored-out copy of an OS .32).

Thanks for any help & advice [sm=thumbup.gif]

Blue.

FHHuber

Increase ply thinckness for firewalls by the 1/2 to full scale of the enlargement. (1/2 scale incraese... if doubling the size of the model, then the thickness increases by 50%)

The wing ribs will probably already be 1/16 sheet... you will want to increase the number of wing ribs to maintain about the same spacing.

Increase the main spars by the scale increase in the chordwize dimension. The rib height dimension is usually increased 1/2 as much as scale change would say. It takes some building experience to determine how much to change the spar thickness... so if in doubt make it the heavier size. THIS is the part that will fail if you size it incorrectly.

Often you will have to add "shear web" between upper and lower spars when enlarging a design thatdid not have the web. In that case, you do not increase the spar thickness as much.

Fuselage side thickness (if sheet fuselage) would increase by 1/3 to 1/2 what the scale ratio says.

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That shoud give you something to go by... With the added experience of having built a few aircraft (its not a good idea to scale up plans without experience building a few directy by the plans) you should do fine.

BMatthews

Great answer and hit all the points.

I'll only add that any sheeting should probably stay at 1/16 as well and I'd probably use heavier ribs or thin ply ribs for the engine nacelle ribs to better tranfer the loads into the spars. And perhaps a spruce doubler in the center section leading edge area to form a sort of triangle with the spars to help lock things tighter against the much higer vibration levels.

dicknadine

can only add, put 1/16 shear webs on each side of the spars, at least 3/4 of the way out to the tip, I usualy put on one side grain up and other side grain length wise. its a strong wing. adds couple of onces. but strong. dick

Blue Sky

My Feedback: (1)

Hi guys,

Really sorry I haven't replied sooner! [] For some reason I didn't receive any notification of your posts [sm=confused.gif]

So... Thank you all very much for your advice!

I've only just had chance to get the plans enlarged - it looks big now! LOL So if it's o.k. with you I'll need a day or so to study the plans and then get back, undoubtedly, with some more questions.

BTW I've built several kits and a couple of models from plans, just nothing this big with a built up wing so that's where my uncertainty lies.

Thanks again!
Blue.

Maybe it helps if I check this little box down here that says "Notify me via e-mail when someone replies". Am I fit to be let out alone....?